1. Field of the Invention
The present invention is in the field of chemical mechanical polishing of substrates. More specifically, the present invention relates to a retaining ring for a chemical mechanical polishing apparatus.
2. Discussion of the Prior Art
In the prior art, the integrated circuits, particularly silicon wafers, are typically formed on substrates, by the sequential steps of laying out the conductive, semiconductive or insulative layers. After the deposition step is completed, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the exposed surface of the substrate becomes largely non-planar. At some critical point, the non-planarity of the outer substrate surface creates problems for the next steps (for example, for the photolithographic steps) of the integrated circuit fabrication process. Therefore, the outmost (exposed) substrate surface is periodically planarized.
One of the most accepted methods of planarization is the method of chemical mechanical polishing (CMP). In the CMP planarization method the substrate should be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad. The polishing slurry tends to be abrasive and corrosive, and can damage the mechanical parts inside the carrier head, including a retaining ring. When the thickness of the retaining ring becomes less than a predetermined thickness, it has to be replaced. If the retaining ring is not replaced in a timely manner, the substrate can be severely damaged.
Accordingly, there is a need to have a retaining ring with a triggering mechanism that can provide a signal to the computer program as to when the chemical mechanical polishing (CMP) process has to be stopped in order to replace the worn out retaining ring, after which the CMP process can be resumed without undue damage to the substrate.